Many transistors formed over a glass substrate or the like are manufactured using amorphous silicon, polycrystalline silicon, or the like, as typically seen in liquid crystal display devices. Although transistors including amorphous silicon have low field effect mobility, they can be formed over a larger glass substrate. On the other hand, although transistors including polycrystalline silicon have high field effect mobility, they are not suitable for being formed over a larger glass substrate.
Other than a transistor formed using silicon, a technique in which a transistor is formed using an oxide semiconductor and applied to an electronic device or an optical device has attracted attention. For example, Patent Document 1 and Patent Document 2 disclose a technique in which a transistor is manufactured using zinc oxide or an In—Ga—Zn-based oxide semiconductor as an oxide semiconductor and such a transistor is used as a switching element or the like in a pixel of a display device.
In an oxide semiconductor, part of hydrogen serves as a donor to release electrons as carriers. When the carrier concentration in the oxide semiconductor becomes high, a channel is formed in the transistor without voltage application to a gate. That is, the threshold voltage of the transistor shifts to the negative direction; it becomes difficult to control the threshold voltage.
In Patent Document 3, it is disclosed that when hydrogen is added in an oxide semiconductor film, electrical conductivity of the oxide semiconductor is increased by four to five orders of magnitude approximately. Moreover, it is disclosed that hydrogen is diffused into the oxide semiconductor film from an insulating film which is in contact with the oxide semiconductor film.
Non-patent Document 1 discloses diffusion coefficients of oxygen and hydrogen in amorphous IGZO. Specifically, Non-patent Document 1 discloses that the diffusion coefficient of oxygen at a heat treatment temperature of 100° C. to 400° C. is 2×10−17 cm2 s−1 to 4×10−17 cm2 s−1, which is higher than that in ZnO or SiO2. Non-patent Document 2 discloses diffusion of oxygen in amorphous IGZO at a heat treatment temperature of 300° C. to 450° C.
Non-patent Document 3 discloses that the density of amorphous IGZO is 5.9 g/cm3.